In A Treatise concerning the Principles of Human Knowledge, Berkeley repudiates Locke’s theory of human perception. Where Locke argued that ideas come from one’s experience of an external, material world, Berkeley argued that the world itself is composed only of ideas. Berkeley held that ideas can only resemble other ideas: an idea in the human mind can only resemble an idea in the external world, not a material object. Consequently, Berkeley argued, being necessitates perception by a perceiver.
Going further, Berkeley argued that it is impossible to prove the existence of material objects external to the self, since all knowledge comes through one’s senses and therefore gives only knowledge of those senses. He argued that Locke’s assertion that primary qualities exist in abstraction, and are therefore knowable only through secondary qualities, was mistaken. It is only possible to know the qualities that are immediately perceptible to the human mind. If they are not directly perceptible (as Locke’s primary qualities were), then there is no way to know them and, therefore, we cannot say that they exist.
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Bishop George Berkeley (1685–1753) was born in Thomastown, County Killkenny, Ireland, and attended Trinity College in Dublin. He earned an MA in 1707 and a doctorate in divinity in 1721. He taught Greek, Hebrew, and divinity at Trinity College until 1724, when he was appointed Dean of Derry in the Church of Ireland. In 1728, he moved to Rhode Island with the hopes of starting a college in Bermuda. Funds for the new college did not arrive as expected, and Berkeley was forced to return to London. While in London, he worked at the Foundling Hospital and helped establish a home for abandoned children. He was appointed Bishop of Cloyne in 1734, living there until he retired in 1752.
Berkeley’s brand of empiricism is known as immaterialism (sometimes subjective idealism). Berkeley taught that matter, as an abstract entity, had no existence on its own. Rather, said Berkeley, objects only exist if they are perceived. An apple, for example, is made up of a number of different qualities (color, smell, taste, etc.). If there is no mind to perceive those qualities, taught Berkeley, the apple would not exist. His motto, “to be is to be perceived,” sums up this philosophy. Berkeley’s empiricism supports the Lockean idea that knowledge relies on observation. It departs from Locke in denying the existence of matter as an abstract entity.